U.S. patent application number 12/119746 was filed with the patent office on 2009-11-19 for method of making titanium-based automotive engine valves using a powder metallurgy process.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS, INC.. Invention is credited to Frederick J. Rozario, Shekhar G. Wakade.
Application Number | 20090282675 12/119746 |
Document ID | / |
Family ID | 41314756 |
Filed Date | 2009-11-19 |
United States Patent
Application |
20090282675 |
Kind Code |
A1 |
Rozario; Frederick J. ; et
al. |
November 19, 2009 |
METHOD OF MAKING TITANIUM-BASED AUTOMOTIVE ENGINE VALVES USING A
POWDER METALLURGY PROCESS
Abstract
An automotive engine valve stem, engine valve and method of
producing both. The valve includes a head and a stem joined to the
head. Lightweight, high-temperature materials, such as
titanium-based materials may be used to make up at least a the
majority of the valve. These materials are combined with
fabrication techniques that may vary between the head and the stem,
where at least a part of the valve is made by dynamic magnetic
compaction. While a majority of the stem may be made from a
titanium-based powder material, its tip may be made of a high
strength hardened material, such as a steel alloy. The valve head
may be made by single press and sintering, double press and
sintering, forging and machining, forging and sintering, and
dynamic magnetic compaction and sintering.
Inventors: |
Rozario; Frederick J.;
(Fenton, MI) ; Wakade; Shekhar G.; (Grand Blanc,
MI) |
Correspondence
Address: |
DINSMORE & SHOHL LLP
ONE DAYTON CENTRE, ONE SOUTH MAIN STREET, SUITE 1300
DAYTON
OH
45402
US
|
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS,
INC.
Detroit
MI
|
Family ID: |
41314756 |
Appl. No.: |
12/119746 |
Filed: |
May 13, 2008 |
Current U.S.
Class: |
29/888.4 |
Current CPC
Class: |
Y10T 29/49298 20150115;
F01L 3/02 20130101; B22F 2998/10 20130101; F01L 2301/00 20200501;
B22F 7/062 20130101; Y10T 29/4941 20150115; B22F 7/08 20130101;
B22F 2999/00 20130101; Y10T 29/49405 20150115; C22C 14/00 20130101;
B22F 2998/10 20130101; B22F 3/02 20130101; B22F 3/17 20130101; B22F
3/10 20130101; C23C 14/00 20130101; B22F 2999/00 20130101; B22F
3/10 20130101; B22F 2202/05 20130101 |
Class at
Publication: |
29/888.4 |
International
Class: |
B21K 1/22 20060101
B21K001/22 |
Claims
1. A method of fabricating an automotive engine valve stem, said
method comprising: configuring said valve stem to comprise a first
end and a second end opposite said first end such that upon
attachment of said valve stem to a valve head, said first end is
proximal and said second end is distal relative thereto, said valve
stem configured such that at least said first end is made
predominantly from a titanium-based powder material, while said
second end terminates in a tip made predominantly of a material
with at least one of strength and hardness properties that are at
least as great as that of said titanium-based material at an
operating temperature of said valve stem; and forming said valve
stem using dynamic magnetic compaction.
2. The method of claim 1, further comprising forming a
substantially radial lock groove between said first end and said
second end of said valve stem.
3. The method of claim 1, further comprising forming a chamfer at
said tip.
4. The method of claim 1, further comprising depositing a hardening
coating on at least a portion of said valve stem.
5. The method of claim 4, wherein said depositing a hardening
coating on said valve stem comprises using vapor deposition.
6. The method of claim 5, wherein said hardening coating comprises
chromium nitride.
7. The method of claim 1, wherein said material with at least one
of strength and hardness properties that are at least as great as
that of said titanium-based material at an operating temperature of
said valve stem comprises a steel alloy.
8. A method of fabricating an automotive engine valve, said method
comprising: forming a valve stem using dynamic magnetic compaction,
said valve stem comprising a proximal interface end and a distal
end, said distal end defining a tip; forming a titanium-based valve
head; and joining said valve stem to said head.
9. The method of claim 8, wherein said forming said valve head is
selected from the group consisting of single press and sintering,
double press and sintering, forge and sintering and dynamic
magnetic compaction and sintering.
10. The method of claim 8, wherein said sintering is performed in a
controlled atmosphere such that oxygen intake by said valve head is
below 10 parts per million.
11. The method of claim 8, wherein at least a majority of said
valve stem comprises a titanium-based alloy.
12. The method of claim 11, further comprising forming said distal
tip end from a different material from said titanium alloy used in
said at least a majority of said valve stem.
13. The method of claim 12, wherein said different material
comprises a hardenable steel alloy.
14. The method of claim 13, wherein said hardenable steel alloy is
hardened after said valve stem has been joined to said valve
head.
15. The method of claim 8, wherein said joining comprises at least
one of friction welding, diffusion bonding, inertial welding, laser
joining and dynamic magnetic compaction.
16. A titanium-based valve for an internal combustion engine, said
valve comprising: a valve head; and a valve stem connected to said
valve head, said valve stem made by dynamic magnetic compaction and
comprising a first end and a second end opposite said first end
such that said first end is proximal and said second end is distal
relative to said valve head, said valve stem configured such that
at least said first end is made predominantly from a titanium-based
powder material, while said second end terminates in a tip made
predominantly of a material with at least one of strength and
hardness properties that are at least as great as that of said
titanium-based material at an operating temperature of said valve
stem.
17. The valve of claim 16, wherein said valve head is made from a
different titanium-based alloy than said first end of said valve
stem.
18. The valve of claim 16, wherein said tip comprises a hardenable
steel alloy.
19. The valve of claim 16, further comprising a hardening coating
disposed on at least a portion of said valve stem.
20. The valve of claim 16, wherein said valve head is made by
dynamic magnetic compaction and said connection between said stem
and said head is through dynamic magnetic compaction.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to the creation of
automotive engine valves using a powder metallurgy process, and
more particularly to intake and exhaust valves, where at least
portions of each are made by one or more of such processes.
[0002] Improved fuel efficiency is an important goal in automotive
design. One way to achieve this is through the use of lightweight
materials and components. Traditionally, rapidly moving and
reciprocating parts, such as engine intake and exhaust valves, have
been made from refractory materials, such as steels, superalloys or
the like. Such materials, while robust enough to endure the rigors
of the internal combustion process, tend to be heavy. This
additional weight has an ancillary impact on other components, such
as springs, rocker arms, bearings or the like that cooperate with
and must therefore be able to withstand the extra forces imposed by
the valves.
[0003] The introduction of titanium has allowed designers to rely
less on refractory materials, providing much of the structural and
temperature requirements at a fraction of the weight of steels,
superalloys and related refractory materials. Precise additions of
alloying ingredients, such as aluminum, vanadium or the like can be
used to tailor the structural properties of titanium. For example,
the fatigue strength at high temperature for exhaust valve stems
must be high, yet not so much so that cold workability and related
manufacturing is hampered. Likewise, such agents used in the head
portion of an intake valve enhance the strength and hardness; where
the tradeoff between wear resistance and component embrittlement
must be balanced.
[0004] Despite these advantages, titanium has not enjoyed
widespread use in engine valve applications. One significant
drawback to titanium is that it is expensive to manufacture,
especially in light of the differing environmental conditions and
requirements at various locations within the valve, such as the
valve tip, stem and head. For example, the valve head is subjected
to a high temperature environment (up to 1400.degree. Fahrenheit)
over significant durations, which could lead to significant creep
loading. Likewise, the valve stem temperatures are a little lower
(up to 1200.degree. Fahrenheit), but are subjected to significant
camshaft and valve spring forces, where compression, tension, shock
and fatigue strength properties become important. These concerns
are especially relevant to the remote tip region of the valve
stem.
[0005] Traditionally, engine valves have been made by forging
(particularly upset forging) followed by heat treatment and
machining, where a titanium alloy rod material is manufactured from
an ingot of titanium alloy, that is then molded then hot swaged so
as to form a valve shape. Such approaches are labor-intensive, as
well as wasteful of the material. Casting techniques have also been
used; however, mechanical properties have been less than with
forging, and are also not well-suited to using disparate materials
within a single casting. More sophisticated casting techniques,
such as local chilling or controlled microstructure variation
through localized aging can improve the casting, but do so at
increased cost, and are often limited to certain (specifically,
ferrous-based) materials. With conventional powder metallurgy, a
metal alloy powder is compacted to a molded valve shape by cold
isostatic pressing, and then sintered. Residual pores in the
as-sintered body results in lower ductility and fatigue
strength.
[0006] Thus, it is desirable that an improved method of making high
strength, titanium-based components, such as engine valves, be
developed. It is further desirable that different approaches best
tailored to particular parts of an engine valve be used to
manufacture the valve. It is further desirable that an engine valve
made by such method be reliable enough for longer-term use. It is
further desirable that a low-cost powder metallurgy manufacturing
process that minimizes the likelihood of residual porosity
formation be used to make at least portions of such a valve.
BRIEF SUMMARY OF THE INVENTION
[0007] These desires are met by the present invention, wherein
improved high strength titanium engine valves and methods of making
such valves are disclosed. According to a first aspect of the
invention, a method of making an automotive engine valves is
disclosed. The method includes configuring the valve stem to
comprise a first end and a second end opposite the first end such
that upon attachment of the valve stem to a valve head, the first
end is proximal and the second end is distal relative thereto, the
valve stem configured such that at least the first end is made
predominantly from a titanium-based powder material, while the
second end terminates in a tip made predominantly of a ferrous
material with at least one of strength and hardness properties that
are adequate to provide necessary wear resistance at the valve tip
at an operating temperature of the valve stem. By such
construction, this forms a dual material valve stem in a single
step using dynamic magnetic compaction (DMC). By this approach,
premium materials can be selectively applied to take full advantage
of their superior properties while maintaining reasonable
manufacturing costs.
[0008] Optionally, the method further includes forming a
substantially radial lock groove between the first end and the
second end of the valve stem. In the present context, the term
"substantially" refers to an arrangement of elements or features
that, while in theory would be expected to exhibit exact
correspondence or behavior, may, in practice embody something
slightly less than exact. As such, the term denotes the degree by
which a quantitative value, measurement or other related
representation may vary from a stated reference without resulting
in a change in the basic function of the subject matter at issue.
In one particular form, a chamfer can be formed at the tip. In
another option, a hardening coating can be deposited on the valve
stem, with a particular form of the deposition being vapor
deposition. The choice of coating can be based on various
compatibility and environmental concerns. Given the operating
conditions in which an engine valve is expected to operate, coupled
with the use of a titanium-based alloy, chromium nitride (CrN) is
one suitable coating candidate. Regarding the tip material, a steel
alloy is one suitable choice. It can be included in such a way that
it is hardened either before or after the valve stem is joined to a
valve head. In another option, the ferrous tip can be hardened
later on using conventional methods, such as induction heating.
[0009] According to another aspect of the invention, a method of
forming an automotive engine valve is disclosed. The valve includes
a head that is joined to a proximal end of a stem, where a distal
end of the stem defines a tip that is hardened relative to the head
and remainder of the stem. As with the previous aspect, one
significant advantage over current methods of manufacture is that
post-forming heat treatment and machining is not required, due to
its near net shape and high quality surface finish. The method
includes forming a valve stem using DMC, forming a titanium-based
valve head and joining the stem to the head.
[0010] Optionally, the valve head can be formed from one of various
techniques, including single press and sintering, double press and
sintering, forge and sintering and DMC and sintering. In another
option, the sintering is performed in a controlled atmosphere such
that oxygen intake by the compacted material (of the valve head,
for example) is below ten parts per million. In yet another
optional feature, at least a majority of the valve stem is made
from a titanium-based alloy, while the distal tip end may be made
from a different material from the titanium alloy used in the
remainder of the valve stem. In one form, the different material
may be a hardenable steel alloy. This alloy may be hardened either
prior to or after the valve stem has been joined to the valve head.
The joining of the stem to the head may be achieved by friction
welding, diffusion bonding, inertial welding or laser joining under
a protective atmosphere to ensure that the joint strength is
optimized.
[0011] According to yet another aspect of the invention, a
titanium-based valve for an internal combustion engine is
disclosed. The valve includes a valve head connected to a valve
stem, where the valve stem made by DMC as discussed in the previous
aspects. As before, the stem includes a first (proximal) end and a
second (distal) end opposite the first end. In addition, the valve
stem is configured such that at least the first end is made
predominantly from a titanium-based powder material.
[0012] Optionally, the valve head is made from a different
titanium-based alloy than that of the first end of the valve stem.
In another option, the tip is made form a hardenable steel alloy.
As with the previous aspects, a hardening coating may be disposed
on at least a portion of the valve stem. In another option, the
valve head may be made from DMC. In addition, the valve head may be
joined to the valve stem through DMC.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0013] The following detailed description of the present invention
can be best understood when read in conjunction with the following
drawings, where like structure is indicated with like reference
numerals and in which:
[0014] FIG. 1 shows a cutaway view of an automotive cylinder head
with intake and exhaust valves;
[0015] FIGS. 2A through 2E show the various steps associated with
making a titanium valve using forging and machining according to
the prior art;
[0016] FIGS. 3A through 3C show the steps used to make a titanium
valve stem using a DMC process;
[0017] FIG. 4 shows an engine valve made according to the present
invention;
[0018] FIG. 5A shows using DMC to form a valve head; and
[0019] FIG. 5B shows using DMC to join a valve stem to a valve
head.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] Referring initially to FIG. 1, portions of the top of an
automotive engine is shown. Piston 10 reciprocates within a
cylinder in the engine block. A cylinder head 20 includes intake
ports 20A and exhaust ports 20B to convey the incoming air and
spent combustion byproducts, respectively that are produced by a
combustion process taking place between the piston 10 and a spark
plug (not shown) in the cylinder. A cam 40 (which is driven from an
external source, such as a crankshaft (not shown)), upon rotation
about its longitudinal axis, selectively overcomes a bias in spring
50 to force inlet valve 60 and exhaust valve 70 to force open the
intake ports 20A and exhaust ports 20B at the appropriate time. It
will be appreciated that the cam 40 and spring 50 shown in
cooperation with intake valve 60 is also used on exhaust valve 70,
but have been removed from the present figure for clarity.
[0021] Referring next to FIGS. 2A through 2E, a conventional method
of manufacturing an engine valve according to the prior art is
shown. The titanium bar 100 of FIG. 2A is machined to produce a
bloom 110, as shown in FIG. 2B. The forging step of FIG. 2C results
in the formation of a separate head 60A and stem 60B such that the
general shape of intake valve 60 starts to appear, while the raw
machining step of FIG. 2D further refines the shape. Finally, FIG.
2E shows a finished intake valve 60, including head 60A, stem 60B
with tip 60C, joined interface region 60D and radiused fillet 60E.
A radial lock groove 60F is formed between the tip 60C and the stem
60B, which provides a retaining feature between the valve stem and
a valve spring cap. The subject feature may be formed from either
ferrous or titanium material. When the double press approach is
used, the formed part hardness after the first cycle should not be
so high that significant densification in the second molding cycle
is precluded. Although not shown, one or more heat treatments may
be performed during the steps shown in FIGS. 2A through 2E.
[0022] Referring next to FIGS. 3A through 3C and 4, a method of
manufacturing the valve 60 includes forming the valve head 60A
separately from the valve stem 60B and tip 60C. Referring with
particularity to FIG. 3B, DMC takes advantage of the compressive
force of a magnetic field on a powder precursor placed in that
field. Upon imposition of an electric current 460 on coil 360, a
magnetic flux 560 is set up in a normal direction as shown. This in
turn sets up magnetic pressure pulse 660 that acts to impart a
radially inward pressure 760 on the precursor powder 160. In it,
the precursor powder 160 is consolidated into a full density parts
in a very brief amount of time (for example, less than one second).
Referring with additional particularity to FIGS. 3A and 3C, in the
DMC process, the powder 160 is placed in an appropriate vessel
(called an armature or sleeve 260). The powder 160 is compacted
from its initial size in FIG. 3A to form the diametrically smaller
cylindrical stem 60B portion shown in FIG. 3C. Initially, the
cylindrical cavity inside the coil is filled with an appropriate
amount of titanium alloy powder mix, followed by a steel alloy
powder of desired composition on the top. This magnetic pressure
pulse consolidates the composite powder mix at relatively low
temperatures almost instantaneously. In addition, this operation
can (if necessary) be performed in a controlled environment to
avoid contaminating the consolidated material. The uniform pressure
distribution is ideal for forming components made from
uniform-shaped parts, of which an axisymmetric valve stem (such as
valve stem 60B) is but one example.
[0023] Preparation of the stem 60B may include using two different
materials (one for the majority of the stem 60B and another for the
tip 60C) and employing a one step DMC process. In one exemplary
form, the stem 60B can be made from a titanium powder alloy and the
tip 60C is made from a hardened steel alloy. The titanium powder
may include various additives tailored to the end use. In one
example, the titanium alloy may be Ti 6-2-4-2, which includes about
six percent aluminum (Al), two percent tin (Sn), four percent
vanadium (V) and two percent molybdenum (Mo). Further, grain
refining agents (such as boron-containing compounds) can be
included in the powder mix, if deemed desirable.
[0024] In addition to the DMC process which includes the use of
inserts to provide profile, it will be appreciated that other
approaches to forming the valve head 60A may be used, such single
or double press and sintering, forging and sintering, or DMC plus
sintering. The sintering operation can be carried out in a
controlled atmosphere, in this way oxygen and related contamination
intake by the titanium-based powder is kept to a permissible level
(such as below 10 ppm). Solution and aging treatments (for example,
age hardening) can be employed to further improve the mechanical
properties. To even further improve the properties (for example,
wear) of the valve 60, ceramic-based coatings (for example, CrN)
can be applied to select valve 60 portions. Such coatings can be
deposited by methods, such as physical vapor deposition (PVD) known
to those skilled in the art. In one particular form, the coating
can be applied to the valve stem 60B and face of valve head
60A.
[0025] Once each of the head 60A and stem 60B (plus tip 60C) have
been prepared, they can be joined at interface region 60D by one of
various methods. In a first, friction welding in controlled
atmosphere is used. In a second, laser welding and cladding (also
in controlled atmosphere) can be used. In a third approach, a
threaded joint with interference secures the two, while in a
fourth, an interference fit without threads is used. Also in one
embodiment the two sections of the valve, namely head 60A and stem
60B may be sintered separately and then joined, or sintered after
joining.
[0026] Referring next to FIG. 4, the valve 60 of FIG. 2E is shown
in more detail, particularly in the head 60B. In a preferred form,
head 60B is made by one or more powder metallurgy techniques, such
press and sinter, powder forge and sinter or double press and
sinter. Detailed features in the head 60B, such as those associated
with the face, underhead radius, head outer diameter chamfer, cup
or the like, may make it difficult to achieve adequate mechanical
properties through a conventional single press and sinter
operation. In such cases, a double press and sinter approach may be
used as it would improve the overall density and hence the
mechanical properties of the head 60B, over single press and sinter
parts. In addition, a controlled atmosphere may be used to minimize
oxygen intake during the sintering operation.
[0027] Depending on the load and environmental requirements, the
head 60A and stem 60B can made from the same or different
titanium-based powders. Likewise, different powder metallurgical
techniques may be used. The valve stem 60B, by virtue of its
axisymmetric shape, is amenable to formation through the DMC
process. For example, the stem 60B can be made by the DMC process
such that the tip 60C is made using hardenable steel alloy. In one
form, this can be achieved in one step. The valve stem 60B is
preferably made from titanium alloy powder whereas the tip 60C of
the stem 60B is made by using the steel powder which could be
hardened later on. The valve head 60A and the stem 60B could be
made from the same titanium-based alloy or could be made from
different alloys. The benefits of using low cost titanium powders,
powder metallurgical technique and near net shape would reduce the
component cost over the forged titanium valves made from wrought
alloys.
[0028] The valve head 60A can be joined to the stem 60B by friction
welding or any mechanical interlocking method or by laser joining
method. Likewise, the sections may also be joined by DMC
processing. Moreover, each of these parts of valve 60 may be made
from DMC processing. Referring next to FIGS. 5A and 5B, the DMC
process may be used in different ways in general, and in at least
two different particular ways as it relates to manufacturing valve
head 60A and valve stem 60B. With regard to the former, parts can
be formed from powder metal using DMC magnetic compaction, while in
the latter to join parts using DMC compressive deformation to
produce interference fits.
[0029] Referring with particularity to FIG. 5A, inserts 860 are
placed within a sacrificial copper sleeve 260A that is used to
define a generally axisymmetric mold in the shape of the head 60A
and radiused fillet 60E. Sleeve 260A is deformed by an imposed
magnetic field (generally similar to those shown and described in
conjunction with FIGS. 3A through 3C) to create the compressive
forces for powder compaction, which results in formation of a
"green" or un-sintered valve head 60A, after which conventional
sintering, machining and related finishing steps may be employed.
The radiused fillet 60E formed may or may not need further
machining depending upon the design specifics, and may, in another
form (not shown) be formed as an angled corner instead of the
fillet. The plates include a lower plate 960 and upper plate 1060
that includes a center core rod 1160. The sidewalls are made up of
coil 360 as shown in FIGS. 3A through 3C. The precursor powder 160
is placed within the voids left between the plates 960, 1060,
center core rod 1160 and inserts 860 and processed in a manner
generally similar to that discussed in conjunction with FIGS. 3A
through 3C and 4.
[0030] Referring with particularity to FIG. 5B, once the head 60A
and the stem 60B are formed separately using the DMC process for
magnetic compaction, a second DMC operation can be used to join the
two through an interference fit can be employed. As shown, two
previously formed "green" parts (i.e., the valve stem 60B and valve
head 60A) could be joined using the DMC process, where a sleeve
260B is placed concentrically around the interface region 60D.
Flange 1260 can be used to remove the sleeve 260B once the
compaction process has been completed. As can be seen from a
comparison of the sleeves 260A and 260B in their respective
figures, both the starting dimensions and the shapes are different.
Specifically, sleeve 260B would be smaller, and would also include
the aforementioned flange 1260. In other regards, the two sleeves
260A and 260B are generally similar in that they both function as
sacrificial (i.e., deformable) carriers of electric current that is
used to effect the DMC process.
[0031] As discussed above, once the head 60A and stem 60B have been
joined, additional processing (such as minimal machining) can be
done. Moreover, protective coatings, for example, CrN, can be
applied. In another form, the head 60A can be made from a
conventional process, such as forging. Such an operation does not
preclude the use of the DMC process to join the head 60A to the
stem 60B. In friction welding as well as in case of laser joining,
the interface between the head 60A and the stem 60B may essentially
be flat without the special features center core rod 1160 and
interface region 60D.
[0032] While certain representative embodiments and details have
been shown for purposes of illustrating the invention, it will be
apparent to those skilled in the art that various changes may be
made without departing from the scope of the invention, which is
defined in the appended claims.
* * * * *